Fareed Iqbal Siddiqui's dissertation
by
Fareed Iqbal Siddiqui, Ph.D.
University of Texas at Austin, 1996
Supervisor: Larry W. Lake
Understanding hydrocarbon migration and trapping is important,
since it can mean the difference between success and failure in
exploration projects. Our current understanding is that a capillary
pressure change between the seal and the carrier bed is the main factor
responsible for the trapping. This theory uses capillary pressure
gradients under static (no flow) conditions to define the maximum amounts
of hydrocarbon that can be trapped under a particular seal. It assumes
that at the very low flow rates encountered in secondary migration viscous
pressure drops are negligible.
This research shows that even at low flow rates the viscous
pressure drops may not be neglected. Trapping at a boundary is a result
of complex interplay among buoyancy, viscous, and capillary forces. A
theory that includes the effect of all of these forces on the trapping of
hydrocarbons is presented. First the effects of the viscous forces are isolated
by neglecting capillary pressure and solving by the method of
characteristics. The results show that viscous forces alone can cause
trapping of hydrocarbons at a heterogeneity boundary. The effects of
capillary pressure are then added and UTCHEM simulator is used to study
the combined effect of viscous and capillary forces. The results show that
even at very low flow rates, viscous pressure drops are not negligible and
that pressure gradients within phases can be substantially different from
relative permeabilities at extreme saturations. The results also show that
neglecting viscous pressure drops may lead to underestimating the
capacity of a seal. The conditions at which a particular seal will leak are
also defined.
The method of characteristics solution is extended to include the
effects of capillary pressures. The results of this research show that it is
possible to classify seals into static seals and dynamic seals based on
the flux rate from the source rock, the capillary pressure curves for the
reservoir and the seal. Static seals conform to the conventional theory. Dynamic
seals, on the other hand, can show substantially more trapping than static
seals. In both cases, it is possible to associate a time scale to the
accumulation and indicate explanations to several other features
commonly observed in secondary migration.
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